YAESU FT-840

I AM USING A COBRA 300 OHM FED ANTENNA FED TO AN EXTERNAL ANTENNA TUNER. WHEN I RAISE THE POWER TO 100 WATTS THE RADIO GOES BACK TO RECEIVE MODE WHEN KEYED. MY METER IS SHOWING NO REFLECTED POWER BACK TO THE RADIO, THE SWR IS GOOD.
THE RADIO AND TURNER ARE GROUNDED TO A COLD WATER PIPE

It will show that the rig/power supply can deliver 100 watts. Sometimes, a 20 amp supply will work--but you may need a 25 amp supply to get enough current on some bands. If that is the case, you may need to get a bigger power supply. Or, you may find that 50 or 80 watts is all you need, in which case you may not need a new power supply.

You could have an RF feedback issue from the antenna being too close to the radio, along with cables connecting the rig acting as an antenna to pick up RF.

I would try improving the ground system. You may be experiencing RF feedback (hot chassis), which is a sign of a poor ground system. I would imagine you're having this trouble on 10 meters?

A cold water pipe can be used as a ground but it would not to be good engineering practice. I have also talked with many who have used a cold water pipe, but were not certain the entire path the cold water traveled was metallic.